Recent progress on vortex fluidic synthesis of carbon nanomaterials

Abstract

Carbon nanomaterials with tunable shapes, morphologies, and sizes have attracted considerable attention owing to their specific physical and chemical properties such as excellent optical and thermal properties, electrical conductivity, and high mechanical strength. Nanomaterial synthesis using dynamic thin films within vortex fluidic devices (VFDs) has many benefits, including a large surface-to-volume ratio of thin films, product uniformity, economic feasibility, environmental sustainability, decreased reaction time, precise temperature and time control, safety improvement, and scalability. This review summarizes recent advancements in the fabrication of various carbon nanomaterials using VFDs, ranging from two-dimensional and three-dimensional (graphene and graphite, respectively) and one-dimensional (carbon nanotubes) to zero-dimensional (fullerene) structures, with controllable sizes, morphologies. Further, this review presents composites with other nanomaterials and metals and the applications of these hybrid materials.

KEYWORDS:

• Vortex fluidic device
• carbon
• nanomaterials
• flow
• fluid
• graphene

1. Introduction

Carbon is an inimitable and vital element owing to its ability to adopt different hybridization states (sp, sp², and sp³) and form strong covalent bonds with other carbon atoms [1–4]. It also covalently bonds with various nonmetallic elements, affording structures ranging from small molecules to long chains [5]. Covalent bonding between carbon atoms allows it to form different morphologies, known as the allotropes of carbon, some of which are in the nanometre range. Different types of nanocarbons include activated carbon, carbon nanotubes (CNTs), fullerenes (such as C₆₀ and C₇₀), graphenes, graphene scrolls, and nanodiamonds, (Figure 1). These carbon nanomaterials can exhibit remarkable properties, including high electrical and thermal conductivity, high structural stability, high mechanical strength, and high surface area and outstanding photoluminescent properties [4,6]. These properties render carbon nanomaterials useful for diverse applications, such as drug delivery, solar voltaic cells, thin-film transistors, photovoltaics, energy storage, supercapacitors, biosensors, photothermal therapy, and heat dissipation [7–10].

Figure 1. Structural illustration of allotropes of carbon nanomaterials with sp² and sp³ hybridization [11].

Owing to their remarkable properties, the development of novel fabrication techniques for carbon nanomaterials using both “top down” and “bottom up” methods while controlling their formation and minimizing waste generation is an important scientific endeavour. Several reviews have already summarized the significant advancements made in the traditional methods used for synthesizing various nanocarbon materials [1,12–14]. However, numerous challenges warrant consideration, particularly in terms of the processing cost, yields, and waste streams resulting from the use of toxic chemicals and reagents, which include surfactants that contaminate the product. Some of these challenges may be overcome using a continuous flow thin-film microfluidic platform for developing novel methods to fabricate nanocarbons of various forms and dimensions, along with studying their properties for potential applications.

C. L. Raston invented and designed a vortex fluidic device (VFD), as published in 2012 [15]. This novel approach in microfluidics has diverse applications, including organic synthesis, the study of the structure of self-organized systems, and nanomaterial preparations. A VFD comprises a borosilicate glass or a quartz tube with outer and inner diameters (OD and ID) of 20 and 17.5 mm, respectively. The tube is open at one end and can rotate at speeds of 2000–10000 rpm, which can generate a thin film of liquid along the tube, depending on the volume present. The tilt angle, θ, of the VFD tube can be readily changed from 0° to 90°, as shown in Figure 2a. The addition of the reactants and collection of the reaction products differ according to the mode of processing. A VFD has two modes of operation, known as the continuous flow mode and confined flow mode (Figure 2b). In the continuous flow mode, solutions are delivered through jet feeds to the base or at strategic positions within the tube with variations in the liquid flow rates along with the rotational speed and inclination angle. In the confined mode, a finite volume of liquid is added to a tube, which is spun across the rotational landscape with variations in the tilt angle. Both operational modes generate dynamic thin films with a thickness ≥200 μm [16,17]. The uniformity and thickness of the dynamic thin film depends on the flow rate, θ, residence time, and rotational speed of the tube, with the film extending to the upper lip where the liquid is forced out of the tube. In the confined mode, the height of the film depends on the amount of liquid, rotational speed, and tilt angle. Both modes of operation generate shear stress, i.e. induced mechanical energy; however, the nature of fluid flow is inherently complex and involves eddies from Faraday waves with associated pressure fluctuations, contributions from Stewartson and Ekman layers, and viscous drag owing to the whirling of the liquid within the tube [17,18]. The VFD design lends itself to applied fields around the tube such that molecules are treated similarly as they move along the tube. The thickness and large surface area of the film are therefore important. For example, photoredox reactions greatly benefit from VFD processing, which considerably improves the conversion up to 96% and can shroud the rotating tube with light-emitting diodes (LEDs) [19]. High mass and heat transfer are important characteristics of a dynamic thin film in a VFD.

The VFD is a continuous flow thin film platform that has been widely used till date. Other than nanomaterial syntheses, it has shown versatility in many fields that harness the tunable shear stress generated by the varying VFD rotational speed. Compared to conventional methods, this continuous-flow microfluidic device has proven advantageous in accelerating chemical reactions [20] by enhancing reaction selectivity [16], fabricating drug formulation [21], controlling the crystallization of nanoparticles [22], and fabricating composite materials [23].

This review presents an overview of the VFD. We also summarize recent developments in the synthesis of carbon nanomaterials, including CNTs, graphene oxide, graphene, fullerene, composites with metals, and hybrids with other carbon materials, using VFDs and their applications. The physical and chemical properties of these nanomaterials, such as shapes, sizes, structures, and surface modifications, can be precisely tuned by a VFD Figure 2.

Figure 2. (a) A photograph and scheme of the vortex fluidic device (VFD), (b) showing its confined and continuous flow modes of operation. (c) The fluid flow behaviour in the VFD, including the spinning top, double-helical flow, and spicular flow [24].

2. Processing carbon nanomaterials using VFD

2.1. Graphene and graphene oxide

Exfoliation graphite to graphene sheets was first successfully achieved using a VFD with N-methyl-pyrrolidone (NMP) as the solvent [15]. This study, published in 2012, was the first work on VFD processing and featured a 10-mm-diameter glass tube, with most of the processing in a 20-mm OD tube. The VFD was operated in the confined mode, with a rotational speed of 7000 rpm and an optimal tilt angle of 45°. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) indicated the presence of monolayer and multilayer graphene (Figure 3a), with the sheets having a lateral size of ∼1 mm. The height profile of exfoliated graphene sheets, examined using AFM, revealed a height of ∼1 nm.

Figure 3. (a) TEM and AFM images of exfoliated graphene sheets produced using a VFD. (b) Schematic of VFD for the reduction of GO suspended in water, with UV irradiation and a nitrogen atmosphere, along with AFM images of GO and rGO with their conductivity and sheet resistance. (c) Schematic and photograph of plasma generation within a VFD. SEM images of air and nitrogen plasma–treated GO. (d) The chemical structure of GO and TPE-2BA (AIE fluorogen), the relation between VFD speed and fluorescence (FL) intensity of GO/TPE-2BA at the excitation wavelength of 310 nm, and the concentration of GO relative to the FL intensity of TPE-2BA [15,25,35,36].

Graphene oxide (GO) is a flat sheet of graphene with oxygen functional groups along the edges as well as on both sides, as described by Lerf et al. [25,26] GO can be stabilized using organic solvents and can also form a colloidal suspension in water [6,27–29]. Reduced graphene oxide (rGO) can be produced from GO using many routes, including chemical, physical, and thermal methods [4,27,30–33]. A VFD can be used to produce rGO under a continuous flow mode at room temperature [34]. Briefly, GO dispersed in water is introduced to a VFD under continuous flow in the absence of harsh reducing agents, with simultaneous UV irradiation (λ = 254 nm, 20 W) and in a nitrogen atmosphere (Figure 3b). This process removes the oxygen functional groups in GO, affording uniform rGO yield. The resulting rGO is comparable to that formed using other processing routes, with a film of the material having a remarkably high conductivity of 2 × 10⁴ S/cm and resistance of 2.2 × 10⁵ Ω [34].

The Raston group also developed a VFD wherein a plasma is generated over the dynamic thin liquid film (Figure 3c) [35]. The species in the plasma are drawn into the thin film as a high mass transfer process at the plasma–liquid interface, and the nanomaterials can be modified depending on the choice of gas (air or nitrogen), as established for GO. SEM and AFM were used to investigate the morphology of GO and understand the effects of plasma treatment. Plasma treatment of GO sheets in air induced morphological changes in the form of pronounced ripples that were compared with a flat control. The average roughness of the treated GO increased by a factor of 5 from that of the control. Treating GO with a nitrogen plasma caused a clustering of the GO sheets. Currently, the dynamic thin film within the VFD is a cost-effective method for fabricating GO/aggregation-induced emission (AIE) and exhibiting high fluorescence [36]. Comparing these results with convectional batch processing, the VFD product was approximately 4– 14 times brighter (Figure 3d). The amount of fluorescence depends on factors such as rotational speed, water fraction, and GO concentration [36].

2.2. Scrolling of graphene and graphene oxide

Two-dimensional (2D) graphene exhibits high stiffness; however, it can be rolled into three-dimensional (3D) graphene scrolls, wherein the diameter of the scrolls can be modified by varying the fabrication method. In this way, graphene scrolls potentially have several applications, such as in hydrogen storage and supercapacitors [37,38].

Alharbi et al. [18] reported an efficient method for fabricating graphene oxide scrolls (GOS) with a high yield. Their method uses a VFD with a neodymium-doped yttrium aluminum garnet (Nd:YAG) 1064-nm pulsed laser operating at 250 mJ. A typical process begins with a colloidal suspension of GO sheets in water, as shown in Figure 4a, followed by systematically exploring the VFD parameter space to find the optimum GOS fabrication conditions. This includes varying the GO concentration, device rotational speed, laser power, and flow rates. Importantly, this method is scalable. For the highest conversion to GOS under continuous flow, the optimized parameters are as follows: a 0.2-mg/mL aqueous suspension of GO, rotational speed of 4000 rpm, flow rate of 0.45 mL/min⁻¹_, and laser operating at 250 mJ. GOS forms because of the shear stress and complex fluid dynamics in the thin film in the VFD. As the liquid tries to accelerate up the VFD tube and is pulled down by gravity, pressure waves are induced by rotational speed. Coupling pressure waves and shear stress with vibrational energy in the GO sheets resulting from laser irradiation likely facilitates the formation of GOS.

Figure 4. (a) Experimental setup and SEM image of fabricated GOS in water under flow. (b) Graphene scrolls from graphite dispersed in toluene and water (1:1) in a VFD (confined mode), with SEM images of the scrolls. (c) SEM images of GO in DMF processing in a VFD under flow, resulting in (i) scroll structures at 4000 rpm, (ii) crumbled graphene at 5000 rpm, and (iii) flat sheets at 8000 rpm; at 5.5000 rpm, GO transformed into ∼100 nm spheroidal particles. By changing the rotational speed, we can cycle between the three structures of GO [18,24,39].

VFD can synthesize graphene scrolls directly from graphite flakes [39]. Scroll formation occurs at room temperature and involves a combination of interfacial tension from the immiscible solvent of toluene and water. The optimal parameters for generating graphene scrolls directly from graphite includes a concentration of 0.5 mg/mL of graphite in a solution of toluene and water at 1:1, with an average lattice spacing of 0.367 nm for the scrolls (Figure 4b) [39]. The underlying mechanism involves the generation of shear stress within the dynamic thin films in the VFD that facilitates the shearing and bending of the graphene sheets, thus forming the scrolls.

Recently, GOS was fabricated using a single solvent system of N, N-dimethylformamide (DMF) under flow in the absence of an external laser and any immiscible solvents unlike in our previously established work. The scrolls were obtained at rotational speeds of 4000 rpm. GOS can be manipulated based on the tube speed: scroll formation occurs at 4000 rpm, structures crumble into globular particles at 5000 rpm, and flat sheets are formed at 8000 rpm (Figure 4c). Cycling between these three forms of GO can be achieved easily by changing the rotational speed of the device [24].

Importantly, these results help understand the response of the dynamic fluid flow in a VFD. Based on the GO manipulation study and other studies such as mixing time profiles and changing the film thickness and temperatures [24]. We know that the dynamic fluid flow in a VFD shows three topologies: spinning top flow, spicular flow, and double-helical flow at low, intermediate, and high speeds, respectively. GOS formation occurs at 4000 rpm owing to the spinning top flow; on increasing the speed to 5000 rpm, the GO sheets collapse and form globular shapes corresponding to the spicular flow. At high speeds of 8000 rpm, GO sheets appear as flat sheets owing to resonant vibrational eddy currents caused by Faraday waves, forming a double-helical flow. Interestingly, at 5.5000 rpm, the GO sheets are shredded into ∼100 nm particles because of the fast interconversion between the proposed double-helical and specular flows, as shown in Figure 4c [24].

2.3. Slicing carbon nanotubes

Slicing of CNTs using a VFD was first achieved in 2016 by Vimalanathan et al. [40] This involves simultaneous and rapid irradiation of the liquid in the glass tube with a pulsed Nd:YAG crystal laser operating at 1064 nm (Figure 5a). The VFD was operated at an inclination angle of 45°, which is now recognized as the optimal angle for general processing using a VFD, and a rotational speed of 7500 rpm. The advantages of this method over conventional methods are the avoidance of using toxic reagents and harsh chemicals as well as the lack of surface defects in the resulting short tubes. The dynamic thin film in the VFD can slice all the three types of CNTs: multi-, double-, and single-walled CNTs (MWCNTs, DWCNTs, and SWCNTs, respectively). The lengths of the sliced CNTs, as determined by AFM, were approximately 190, 160, and 171 nm for SWCNTs, DWCNTs, and MWCNTs, respectively [40]. In addition, CNT slicing occurred for both modes of VFD operation, namely the continuous flow and confined modes, with the confined mode having better control over the length of SWCNTs than for DWCNTs and MWCNTs.

Figure 5. (a) Experimental setup for the lateral slicing CNTs in a VFD irradiated with a pulsed laser at 1064 nm, with AFM images of the sliced CNTs. (b) AFM images of sliced SWCNTs; (c) schematic showing vertically aligned sliced short MWCNTs and AFM images of a fresh silicon substrate (before dipping), after dipping for 30, 60, and 120 min. (d) Schematic of fabrication of CDs using a VFD, with the corresponding TEM and AFM images. (f) Unzipping MWCNTs and high-resolution TEM (HRTEM) images [40–45].

Based on previous research on slicing CNTs, [40] Alharbi et al. [41] used the advantages of laser-irradiated dynamic thin films in VFDs for controlling the length distribution of sliced SWCNTs. The SWCNT length was controlled by varying the power of the pulsed laser as well as other parameters such as the device speed, flow rate, concentration of the product, and solvent choice. Laser energies of 250, 400, and 600 mJ produced 700, 300, and 80 nm SWCNT length distributions, respectively (Figure 5b). This strategy is suitable for slicing and controlling the length of the SWCNTs without sidewall damage. Furthermore, this method is scalable as it is done under flow conditions and avoids using surfactants or any other harsh chemicals. Similarly, the length of sliced MWCNTs was controlled using a VFD coupled with a 1064-nm pulse laser. The MWCNTs were successfully sliced with three length distributions, centred at 550 ± 1.4 nm, 300 ± 1.8 nm, and 75 ± 2.1 nm [42].

In addition to controlling the length of sliced MWCNTs, Alharbi et al. [42] reported that for sliced MWCNTs with shorter lengths (75 ± 2.1 nm), the surface density coverage can be controlled via a straightforward dipping and rinsing process, allowing them to vertically self-assemble on a substrate (Figure 5c). This was determined by dipping a silicon substrate in a short MWCNT solution for different durations of 120, 60, and 30 min. AFM measurements were taken before and after the immersing process. The silicon substrate appeared as flat and devoid of any morphology before dipping into the solution. After the substrate was immersed into the short MWCNT solution for 30 min, the attached sliced MWCNTs were obvious, with the coverage level increasing with the immersion time. Importantly, two factors were critical for vertically self-assembled sliced MWCNTs and controlling their density on the substrate: the MWCNT length and immersion time.

Luo et al. [43] reported a simple, environmentally friendly preparation of water-soluble carbon nanodots (CDs) from MWCNTs under flow mode using a VFD. The preparation involved the delivery of a MWCNT dispersion in 30% H₂O₂ using a magnetically stirred syringe to a VFD tube that was simultaneously irradiated with a Q-switch Nd:YAG (1064 nm) laser. The resulting CDs from controlled oxidation of the carbon material were characterized using different techniques, including TEM and AFM. The fabricated CDs were spherical in shape with a diameter of 6 nm, which is in agreement with the suggested structure shown in Figure 5d, comprising several layers of graphene sheets.

Recently, considerable efforts have been made to find new ways for slicing CNTs using a VFD without axillary reagents or surfactants and avoiding the use of any external fields. Raston et al. developed a simple method for the controlled slicing and disentangling of both multi- and single-walled CNTs. The method is environmentally friendly as uses the mechanical energy generated in a biphasic immiscible mixture of (1:1) water and o-xylene using flow mode in the VFD. Importantly, a high yield 93% can be obtained using this simple method under flow conditions. The length distributions were centred at 500 and 490 nm and for MWCNTs and SWCNTs, respectively (Figure 5e) [44].

Recently, MWCNTs were successfully unzipped using continuous flow VFD operational mode at ambient temperatures, with a high yield of 75%. The process uses aqueous H₂O₂ and avoids the use of harsh chemicals. This is an inexpensive, scalable method to unzip MWCNTs, [45] as shown in Figure 5f.

2.4. Fabrication of carbon nanorings

The bending of straight SWCNTs in a controlled manner to form toroidal or nanoring structures has many potential applications, such as flexible electronics and small electromagnetic devices [46,47]. Vimalanthan et al. [48] developed a novel VFD-mediated approach to prepare single-wall carbon nanorings of various morphologies with control over their diameter, which was characterized using AFM and TEM. The process used an immiscible solvent system of toluene and water, with an optimal rotational speed of 7500 rpm and a tilt angle of 45^o. The nanorings comprised coiled SWCNTs with diameters of 300–700 nm for a 20 mm OD glass tube (ID 17.5 mm; Figure 6a) and 100–200 nm for a 10 mm OD glass tube (ID 8.5 mm). This was the first report to gain insights into the effects of tube diameter variations [48]. The results clearly show that SWCNTs bend owing to shear stress in the VFD, which is important when considering the effects of pulsed lasers for slicing SWCNTs, DWCNTs, and MWCNTs, as discussed above. The vibrational energy from the absorption of near-infrared (NIR) radiation causes C–C bond cleavage associated with shear stress–induced bending of the SWCNTs.

Figure 6. (a) Proposed mechanism for fabricating SWCNT nanorings in a VFD. AFM and TEM images of the different nanoring morphologies. (b). SEM, TEM, and HRTEM images with histograms of the diameter and thickness distributions of coiled SWCNT rings, formed in a VFD under flow. (c) MFM image of an SWCNT ring with 50 nm thickness and MFM phase images at various lift heights. AFM and MFM images of SWCNTs with different structures [48,49].

Alharbi et al. has made considerable progress regarding the fabrication of SWCNT rings in the continuous flow mode of a VFD. They successfully achieved a high yield (80%) of ring structures from straight SWCNTs using toluene and water without surfactants or toxic chemicals [49]. The SWCNT rings have a wall thickness of 3–70 nm and average diameter of ∼300 nm (Figure 6b). Magnetic force microscopy (MFM) demonstrated that the magnetic responses of the synthesized rings were strongly dependent on ring thickness. The magnetic response of the SWCNT rings was dependent on the curvature effects; arrangement of heptagons and pentagons in ring structures formed from long SWCNTs; and electronic structure and chirality of the SWCNTs. Additionally, during the continuous flow processing of SWCNTs ring, other morphologies such as cross-lattice and figure-eight shapes were created and showed no detectable magnetic response (Figure 6c) [49].

2.5. Self-assembly of fullerene C₆₀

Various methods have been reported on the control of C₆₀ molecule crystallization to form diverse structures. However, most of the methods developed thus far use surfactants and toxic solvents [50–52]. Vimalanathan et al. developed a method to fabricate hollow tubules of self-assembled fullerene C₆₀ [53] using a VFD without any surfactants, wherein fullerene dissolved in toluene and water was delivered in separate jet feeds under continuous flow conditions, with water acting as an antisolvent (toluene: water, 1:1). The optimization of the process to control the growth and nucleation of the nanostructures of C₆₀ involved systematically varying the processing parameters of the device, initial rotational speed, and inclination of the glass tube, before converting the processing to continuous flow. The hollow nanotubules (Figure 7a) were ∼0.4–3 µm in length with an ID of ∼100–400 nm, formed with a 45^o tilt angle and 7000 rpm tube speed.

Figure 7. (a) Schematic representation and SEM images of C₆₀ nanotubules fabricated in a VFD using a mixture of water and toluene and their selectivity and sensitivity to various solvent molecules. (b) Schematic and SEM images of synthesized C₆₀ cones in a VFD. (c) Self-assembly of crystallized C₆₀ in toluene using shear stress in a VFD at different rotational speeds [24,53,54].

Alsulami et al. reported a simple and effective way to synthesize a cone structure from self-assembled C₆₀ under the continuous mode in a VFD (Figure 7b). The assembled fullerene cones were stable at room temperature for more than 30 d. Briefly, fullerene was dissolved in o-xylene and left for 24 h. DMF was then used as an antisolvent during VFD processing [54]. The resulting cones had a high yield and uniform shape. SEM measurements determined the cone thickness to be 120–310 nm and their diameter to be 0.5–2.5 µm.

A new form of fullerene crystallization was recently achieved in a VFD by dissolving C₆₀ fullerene in toluene only [24]. At rotational speeds < 6000 rpm, spicular structures were generated with a uniform size and number, whereas rods and spicular structures were synthesized at >6000 rpm (Figure 7c). The formation of spicular structures is owing to double-helical fluid flow and spinning top flow. These results, along with others, help understand fluid dynamics within a VFD [24].

2.6. Decorating nanoparticles on carbon nanomaterials

Carbon nanomaterials decorated with metal nanoparticles have received considerable attention owing to their physical and chemical properties such as high surface-to-volume ratio and active surface area [55,56]. Metal oxides on the surface of carbon materials show promising results in many fields, including energy storage, catalysis, sensors, imaging, and therapeutics [29,33,57].

Earlier studies on VFDs have demonstrated that this technology is convenient for decorating carbon nanomaterials with metal nanoparticles. For example, carbon nano onions can be decorated with platinum (Pt) nanoparticles. The size and density of the nanoparticles can be controlled by varying the device parameters, and the process is conducted in an aqueous medium with hydrogen as the reducing agent (Figure 8a). These hybrid materials have excellent electrocatalytic activity [58].

Figure 8. (a-b) Scheme representing and TEM images of the process for decorating carbon nanomaterials (CNMs), including carbon nanotubes and carbon nano onion with Pt/ palladium nanoparticles. (c) Schematic and TEM images of synthesized Fe₃O₄@MWCNT formed in the under continuous flow with their capacitance [58–60].

In a similar way, carbon nano onions were decorated with Pt nanoparticles using ascorbic acid as the reducing agent. Formation of the composite nanomaterials using both types of platinum reduction (hydrogen versus ascorbic acid) relies on the shear stress in the VFD [33]. Furthermore, palladium (Pd) nanoparticles were successfully decorated on SWCNTs using VFDs (Figure 8b) to form a composite material, which effectively combines the electrical properties of SWCNTs with the hydrogen absorption capability of Pd, with uses in sensor applications. The first step of this process is the binding of Pd²⁺ to p-phosphonated calixarenes on the surface of the SWCNTs. Reduction in VFD generates Pd nanoparticles 1–5 nm in diameter. The composite material is effective in sensing hydrogen gas in the range of 0.1%–10% [59].

Alharbi et al. [60] reported a three-step in one process in a VFD to fabricate composites materials of carbon nanotubes (MWCNTs) decorated with iron oxide nanoparticles (Fe₃O₄@MWCNT). This operation includes: in situ superparamagnetic magnetite nanoparticles generation by irradiating iron with a 1064-nm pulsed laser over the dynamic thin film in the microfluidic platform; slicing of MWCNTs; and decorating the MWCNT surface with Fe₃O₄ magnetite nanoparticles (Figure 8c). An active electrode was made from the composite material Fe₃O₄@MWCNT and used for supercapacitor measurements, with areal capacitances of 1317.7 mF/cm² and 834 F/g and a scan rate of 10 mV/s. Moreover, the Fe₃O₄@MWCNT material showed a higher specific energy of 115.84 Wh/kg at a specific power of 2085 W/kg. This indicates the potential use of this material in energy storage systems.

2.7. Synthesis of composite nanomaterials

Microencapsulation of bacteria provides a protective layer while retaining their function, except for replication, which can clog the matrix [61]. VFDs are promising in this regard [61], with their ability to encapsulate Rhodococcus opacus and Staphylococcus aureus using GO while keeping the cells alive and biologically active (Figure 9a). This process uses a 10-mm OD tube and a 1:1 mixture of 0.1 mg/mL of GO and 0.5 mL of the bacterial solution using the confined mode at a rotational speed of 5000 − 8000 rpm for 1 min after mixing the GO and bacterial solutions [61]. The ability to encapsulate bacteria with different spherical and rod-shaped morphologies is related to the nature of the fluid flow in the VFD, and part of this review is focused on understanding this.

Figure 9. Schematic representing the fabrication of composite nanomaterials in a VFD, including (a) GO wrapped bacterial cells, (b) GO wrapped algal cells, (c) exfoliated graphene sheet − wrapped algal cells, (d) graphene spheres with their capacitance, and (e) SEM image of polystyrene beads and fullerene C₆₀ [61–65].

A VFD has also been effective in exfoliating multilayered graphene to be used as a material for the partial wrapping of algal cells. Graphite was exfoliated in water were using the confined mode of the VFD with a 10-mm OD tube. The cross-sectional area of the graphite flakes was 7–10 μm and the inclination angle of the tube was optimal at 45°, and the rotational speed was optimal at 7000 rpm for 30 min of processing. The wrapping step involved the addition of a 1 mL of a suspension of wild-type Chlorella Vulgaris, following which the mixture was recycled through the VFD under continuous flow conditions, using a 10 mm OD glass tube (Figure 9b) [62].

Wahid et al. [63] reported the simple fabrication of a biohybrid material using a VFD. The biohybrid material is formed by exfoliating graphite in water to form multiple layers of graphene, which are then used for coating the surface of microalgal cells. The process uses naturally abundant algae and graphite. The reaction medium being water also incorporates the principles of green chemistry. Moreover, this hybrid material is used for wastewater treatment, in which the initial nitrate content is removed within 4 d (Figure 9c).

In the field of hybrid CNTs, Alsulam et al. [64] prepared graphene spheres confining fullerene C₆₀ at room temperature, from a colloidal suspension of graphite in DMF and an o-xylene solution of C₆₀. The processing was conducted in a continuous flow mode and under high shear using a VFD. The composite spheres have controllable diameters that depend on the VFD parameters and are in the range of 1.5–3.5 µm. Additionally, the hybrid material of (graphene @ C₆₀) used in an electrochemical device shows high cycle stability, with a high areal capacitance of 103.4 mF/cm² and maintenance of its capacitances at 24.7 F/g and 86.4 mF/cm² (83.5%) at a high scan rate of 100 mV/s (Figure 9d).

Recently, spicular structures of fullerene C₆₀ over the surface of spherical poly(styrene-co-divinylbenzene) were fabricated using a continuous flow mode in a VFD. The fabricated composite material had a uniform shape and were highly isolated (>98%). This approach could open the door for other applications such as surface modifications of materials, coating small-molecule drugs, macromolecules, and nanoparticles (Figure 9e) [65].

3. Conclusion and future perspective

This review describes how the VFD thin film microfluidic platform has been used to process nanocarbon materials and their composites. The mechanical energy (shear stress), controlled by the dynamic thin films of this microfluidic platform, provides control over the shape, size, and morphology of the CNT, enabling processing with high sustainability. This includes reducing the need for toxic and harsh chemicals by avoiding further processing steps and energy consumption. Moreover, VFD has the potential as a more controllable alternative to the syntheses of various morphologies of nanocarbon materials using both bottom-up and top-down approaches. The techniques developed thus far have changed the way high tensile materials may be worked in a controlled manner, rendering it attractive to various industries by improving the scalability.

There remains a need for further research to advance the field of VFD technology. First, we must understand the mechanisms of fluid flow along the VFD tube. Second, to quickly simplify and optimize reactions in the processing of CNTs, it is necessary to include more theoretical studies and investigate the rotational speeds for all material syntheses. Moreover, the processing optimizations could benefit further from the automated control of VFD processing. Finally, for this technology to become widespread and safe, the availability of VFDs, which may require the 3D printing of essential components, needs to be addressed.

Acknowledgement

TA extends his appreciation to Prof. Colin L. Raston from Flinders university in Australia for his valuable discussions and proofreading this work. As this work a part for my PhD journey, The author thanks Taibah University for funding his scholarship.

Disclosure statement

No potential conflict of interest was reported by the author(s).